The present invention relates to braking systems in general and, in particular, to a friction braking system for a rubber tire transit vehicle. Modern transit vehicles such as used in San Francisco's Bart system and the Orlando and Atlanta airports utilize a combination of dynamic or regenerative braking and friction braking to bring the vehicles to a controlled stop. These modern systems have been described in articles such as "Atlanta Airport People Mover," by Thomas C. Selis, Manager, in the Conference Record of the 28th IEEE Vehicular Technology Group in Denver, Colo., on Mar. 22, 1978, and also in an article "Recent Applications of Microprocessor Technology to People Mover Systems," by Michael P. McDonald et al., in the Conference Record of the 29th IEEE Vehicular Technology Group Conference in Chicago, Ill., Mar. 28, 1979.
U.S. Pat. No. 3,398,992 issued to Joseph C. Littman, dated Aug. 27, 1968, discloses a brake control system for hydraulic brakes on a trailer connected to a towing vehicle. The Littman patent teaches variation of the pressure of the fluid in the hydraulic system to vary the degree of braking. U.S. Pat. No., 4,384,330 issued to Matsuda et al., dated May 17, 1983, discloses a brake control system for an automotive vehicle for controlling application release of brake pressure in order to prevent the vehicle from skidding. Another deceleration control system is disclosed in U.S. Pat. No. 3,751,116 issued to Thomas H. Engle, dated Aug. 7, 1983, which discloses a railway brake controller which modulates the train line brake-controlling signal as required to maintain balance between a command force indicative of a desired rate of retardation and a feedback force developed by a liquid-filled column subject to the actual rate of retardation. Another brake control apparatus is disclosed in U.S. Pat. No. 4,410,154 issued to Thomas C. Matty, dated Oct. 18, 1983, and assigned to the present assignee, which is incorporated herein by reference. The Matty reference discloses a transit vehicle control apparatus which determines a safe brake velocity for a vehicle in relation to a speed control relationship including roadway system design deceleration, the deceleration of the vehicle in relation to inertial space and the deceleration of the vehicle in relation to the roadway. The Matty patent teaches the use of microprocessor-based technology to accomplish braking control.
The typical transit vehicle or "people mover" as part of the braking system includes friction drum brakes of the same type that are used on tractor trailers and which are well known in the art. The friction brakes are controlled by friction brake actuators which are typically integral units having a fail-safe design such that if the actuators suffers loss of air pressure, a spring causes the friction brakes to engage. Referring to FIG. 6A, there is shown a typical deceleration curve upon the application of the emergency brake spring-activated system. As apparent from graph 6A, there is typically an initial spike or a high deceleration rate which may produce an uncomfortable jerk for passengers on the transit vehicle. This spike occurs as a result of the service brakes which are air-activated being fully applied during an emergency stop having a faster response time than the spring-activated system. An accelerometer which is typically connected in circuit with the service brakes is typically too slow to retard the onset of the service brakes even though sufficient deceleration is being supplied by the spring activated system. A typical prior art system producing this response is shown in FIG. 4.